Cables for electric power, control and communication lines are run underground in order to protect them from above-ground elements and from the interference and damage they would suffer when installed above the ground or on poles or other structures. Conduits, also called ducts, run underground for such cables should be parallel to each other and spaced apart from each other in a controlled manner in order to minimize any electrical interference. This spacing also acts to dissipate the heat generated by transmission of electric power and electric signals in the cables. In addition, the conduits and cables should be protected, primarily from digging, whether with hand tools or with mechanized equipment, such as backhoes.
A trench may be dug and conduits placed into the trench at a distance from each other. For example, a series of conduits may be placed side-by-side in the bottom of the trench and separated from each other by removable spacers. Once the conduits are placed, the spacers may be removed and all space between the conduits filled with earth, sand or concrete. Thus, the space is filled with thermally conducting but electrically insulating material.
It is important to fill all the spaces between the conduits no matter which technique is used to space them apart. It is difficult to accomplish this when the conduits are in several vertical layers or tiers. The filler material ideally should be a flowable material, i.e., a material that flows freely downward and sideways in all directions when dispensed into the trench. A more-flowable filler material consists of 50 to 100 lb (about 23 to about 45 kg) Portland cement, 2750 lb (about 1250 kg) of fine sand, and 500 lbs. (about 227 kg) water (maximum) per cubic yard (about 0.765 cubic meters), having a 28-day compressive strength of 50-150 psi (about 0.34 MPa-about 1 MPa). A heavier but still flowable filler material includes a normal weight concrete mix with Portland cement, aggregate having a maximum size of ⅜ inch (about 9-10 mm), and sand and water. The heavier material has an 8 inch (about 203 mm) minimum slump and a 28-day compressive strength of 3000 psi (about 21 MPa). The ability to spread and fill the entire space is needed for good heat transfer and thermal conductivity.
One way to insure even spacing between conduits for power and communications cables is to fabricate banks of ducts which are separated by conduit spacers, also known as duct spacers. The duct banks are then encased in concrete or flowable fill. After the concrete has hydrated or set, cables are pulled through the conduits. The concrete provides a heat transfer medium for conducting heat to the surface, normally the ground surface, and also protects the cables from moisture, rodents and any contractors attempting to dig in the immediate vicinity of the duct bank.
Fabrication of a duct bank typically requires preparing an assembly of conduits and spacers in a trench and then encasing the duct bank in concrete or flowable fill. One method of assembling the spacers is to simply place conduits into bores prepared in a first layer of one or more spacers, and then to place additional spacers and conduit atop the bottom layer. A sturdier assembly may be made by positively locking the conduits into the spacers and by locking the spacers themselves together. Spacers, such as those made by Underground Devices, Inc., Northbrook, Ill., typically include attachment features that allow interlocking of the spacers in a horizontal direction. The spacers are then assembled with the conduits. The conduits themselves provide vertical interlocking with the spacers. This combination of horizontal and vertical interlocking thus joins the spacers and conduits into a duct bank, an assembly that is placed into a trench, and as noted above, is then backfilled to provide mechanical stability and heat transfer.
One problem that is encountered with such duct banks is the phenomenon of the duct bank floating or moving when backfilling the trench with concrete or flowable fill. The duct bank includes conduits or ducts and spacers that have a specific gravity that is far less than concrete or flowable fill. Therefore, when concrete is poured into the trench, the duct bank may move and will float unless it is restrained.
In order to restrain the duct bank and hold it in place, an apparatus such as a conduit retainer may be used, as described in U.S. Pat. Nos. 7,223,052; 7,614,828, and in U.S. Pat. Appl. Publ. 2003/0198520. These patent documents describe techniques that use hold-down bars atop the duct bank to secure the duct bank within a trench. The hold-down bars are secured with anchors on the sides of the trench. Other variants of these techniques are also known, as disclosed in the accompanying Information Disclosure Statement. The disadvantage of this technique is the effort required to secure the hold-down bars at sufficient spacing on both sides of the trench. This technique also fails to secure the duct bank from horizontal movement during backfilling, i.e., the duct bank may move unpredictably in a sideways direction within the trench even though it is restrained against vertical movement.
Another technique uses hold-down bars atop the duct bank, the hold-down bars secured with rods driven through the duct bank and into the trench. The hold-down bars are then secured to the securing rods with retaining rings, e.g., retaining rings, such as type-E retaining rings. These retaining rings are secured on top of the hold-down bars to the securing rods around only a portion of their periphery or circumference. The retaining function is often tenuous and may not be reliable. Thus, the hold-down bars themselves may not be securely held.
What is needed is a method and apparatus to reliably secure the duct bank in place in the trench while backfilling takes place. The method and apparatus should be able to withstand the considerable forces applied to the duct bank during backfilling, these forces capable of moving the duct bank itself sideways during backfilling, and also capable of causing the duct bank to float during backfill if the hold-down bars are not reliably secured to the securing rods.